1. Field of the Invention
The invention relates to a method for synchronized shifting or keying of a polarization condition of an optical transmission signal for an optical transmission system having super-heterodyne reception. The invention also relates to an apparatus for the implementation of the method.
2. Description of the Prior Art
In standard optical receivers having super-heterodyne reception such as optical heterodyne receivers, fluctuation of the amplitude of the intermediate frequency signal is dependent upon the relative position of the polarization conditions of the optical data signal and of the optical signal of a local oscillator. For optimum transmission quality, these fluctuations must be kept as low as possible. Common solutions are polarization follow-up with a control loop usually referred to as polarization control disclosed in Electron. Lett. 22 (1986), pp. 1341-1343; polarization quadrature reception, Electron. Lett. 22 (1987), pp. 168-169; and polarization scrambling, Electron. Lett. 23 (1987), pp. 513-514.
Receivers having polarization follow-up, theoretically achieve high sensitivity. However, the outlay for the control circuitry in such receivers is considerably large.
Polarization quadrature receivers represent a reliable alternative, but require a division of optical signals into two sub-signals that are polarized so that they are orthogonal to one another. Each of the sub-signals requires its own receiver. Both methods demand an increased outlay in the receiver. This drawback is problematical in many receiver applications.
In polarization scrambling, as opposed to receivers with polarization control, the increased outlay can be shifted onto the transmitter side. In this method, the polarization condition of the data signal is repeatedly switched between orthogonal states during the bit duration. This allows for the mean value of different polarization conditions to be represented by the intermediate frequency signal. Theoretically, the sensitivity loss in this method compared to the ideal polarization follow-up amounts to at least 3 dB. For high data rates, extremely fast polarization switches are required and the required receiver bandwidth is significantly increased.
It is well-known to have frequency-shifted signals corresponding to data symbols 0 and 1 for a transmission having polarization conditions that are orthogonal to one another. Such a two-filter FSK system is disclosed in Electron. Lett. 24 (1988), pp. 358-360. However, polarization insensitive reception requires high frequency shifting and large receiver bandwidths despite low data rates. Further, this method is only suitable for a two-filter FSK system.
In polarization scrambling, the polarization condition of the optical transmission signal is switched between orthogonal states usually several times during a bit period of the data signal. Moreover the switching signal is usually not synchronized with the data signal.
Prior art optical systems exhibit a dependency on polarization which yields poor transmission quality.